As one of many safety devices for a four-wheel vehicle such as an automobile or a truck, tire pressure drop detecting devices have been invented, and some of them have been put to practical use. There is a great request to detect a tire pressure drop for a vehicle particularly in Europe. The reason for this is that in European countries where expressways are developed, a tire pressure drop of a vehicle which is traveling at high speed is liable to cause a large accident.
Examples of the conventional tire pressure drop detecting devices which have been put to practical use include one for directly measuring tire pressures. In this conventional device, pressure sensors are respectively provided in tires, and the pneumatic pressures of the tires are directly measured by the pressure sensors. The measured tire pressures are applied to a processing device provided on the side of the body of a vehicle. The pressure sensors and the processing device are coupled to each other, for example, electromagnetically, thereby making it possible to mechanically send and receive signals in a non-contact state. The tire pressures processed by the processing device are displayed on a display or the like.
The device so constructed that pressure sensors are provided in the tires has the disadvantage of being very high in cost. In addition, it also has the disadvantage in that errors are liable to occur at the time of signal transmission, although signals are electromagnetically transmitted to the processing device provided on the side of the body of the vehicle from the pressure sensors provided in the tires. Particularly, the vehicle may, in some cases, travel under bad conditions, for example, on a road having a lot of earth magnetism, thus causing many barriers against electromagnetic signal transmission.
Therefore, as another conventional example, a tire pressure drop detecting method for detecting the rotational angular velocities of four tires and detecting a tire whose pneumatic pressure relatively drops on the basis of the rotational angular velocities of the respective tires has been proposed (see Japanese Patent Unexamined Publication No. 63-305011, for example).
In this conventional detecting method, the sum of the rotational angular velocities of a pair of tires on a diagonal line is subtracted from the sum of rotational angular velocities of the other pair of tires on another diagonal line, and a reduced pressure tire is detected if the result of the subtraction is in a range of 0.05% to 0.6% , preferably, in a range of 0.1% to 0.3% of the average value of the two sums.
However, the conventional method of detecting a reduced pressure tire on the basis of the rotational angular velocities of the tires has the following disadvantages. Specifically, in order to obtain an amount of the change in the rotational angular velocity of the reduced pressure tire, the average value of the rotational angular velocities of the four tires is taken as a reference amount, thus detecting the amount of the change in the rotational angular velocity of the reduced pressure tire on the basis of the reference amount. Therefore, the reference amount also includes the rotational angular velocity of the reduced pressure tire, so that the reference amount is not exact and the accuracy of the detection is not high.
Furthermore, the conventional detecting method also has the disadvantage in that the calculation of the rotational angular velocity for each of the tires which is the basis of the detection is not accurately conducted.
More specifically, in the conventional method, each of the tires is provided with a sensor for generating a signal having a frequency proportional to the rotational angular velocity of the tire to count the arising edges or the falling edges of an output signal of the sensor in a measuring period. Since the measuring period and the arising edges or the falling edges of the output signal of the sensor are not synchronized with each other, however, the timing of starting the measuring period and the timing of the arising edges or the falling edges of the output signal of the sensor vary for each measuring period. Therefore, the conventional method has the disadvantage in that an error occurs in the calculation of the rotational angular velocity.
Furthermore, as still another conventional example, a device for detecting a reduced pressure tire on the basis of a wheel speed signal, which is provided with a steering angle sensor and an acceleration sensor to inhibit the reduced pressure tire from being detected while a vehicle is cornering and is being accelerated or decelerated has been proposed (see Japanese Patent Unexamined Publication No. 63-64804).
In the conventional device, however, the steering angle sensor for detecting the steering angle of a handle is indispensable so as to prevent the erroneous detection of the reduced pressure tire in a case where the vehicle is cornering, and a gravity sensor for detecting the acceleration or the deceleration of the vehicle is required. Therefore, the conventional device has the disadvantages of being complicated in construction and being high in cost.